Microbial fuel cells (MFCs) have recently been considered as an alternative and promising way for the direct conversion of the biochemical energy bound to wastewater into electricity without any intermediate step.
A MFC is fundamentally an anaerobic process where bacteria grow in the absence of oxygen in a chamber containing an anode and form a biofilm that covers the anode. To generate electricity, bacteria in the anode chamber degrade organic matter (the influent/fuel) and transfer the electrons to the anode. These electrons pass through an external circuit producing current. The digestion of organic matters occurs in anaerobic environment so that the electrons are not consumed by electron acceptors such as oxygen. The transfer of these electrons occurs via bacterial respiratory enzymes. Protons, created at the anode to maintain a charge balance, migrate through the solution to a cathode where they combine with oxygen and the electrons produced at the anode to form water. Hence, the cathode has to be maintained under aerobic conditions.
MFC can be of two types: 1) a two-chambered MFC, where the anode chamber is anaerobic and the cathode chamber is aerobic (FIG. 1A), or 2) a single-chambered MFC where both electrodes are placed in an anaerobic chamber, with one face of the cathode exposed to the air (FIG. 1B). A proton exchange membrane (PEM), aiming at facilitating the transfer of protons, usually separates the anode from the cathode. The potential difference between the respiratory enzyme and oxygen results in electricity generation.
Microbial electrolysis cells (MECs) are, simply put, modified microbial fuel cells (MFCs) designed to produce hydrogen gas at the cathode. Like an MFC, the MEC is composed of one or two chambers containing an anode and a cathode. Bacteria on the anode catalyze the oxidation of organic substrates, producing electrons and protons. The electrons from the metabolic reactions travel through an external circuit towards the cathode. Protons are transferred to the cathode in the aqueous solution. At the cathode, the electrons and protons recombine to form hydrogen gas. To successfully produce hydrogen gas, an external voltage is applied to overcome thermodynamic barrier, as the direct production of hydrogen from the hydrolysis of organic compounds is not thermodynamically feasible. The hydrogen gas produced by MECs is of high purity compared to other biological hydrogen production processes, negates the use of expensive gas purification techniques.